Need help finding a relay

Discussion in 'General Electronics Chat' started by oblivioncth, Apr 2, 2013.

  1. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Hi, I am pretty experienced with computers, soldering, etc., but I am really just getting started when it comes to understanding electronic components and circuits.

    I came up with a cool idea for adding a feature to dual NAND xbox 360s with the DemoN installed (if anyone here knows about that) and have figured out exactly how I am gonna do it, but I am at a loss for finding the relay I need.

    I need a relay that is powered by a 3.3V source and toggles the power from a SATA power port on the console between two 2.5" "Laptop" hard drives.

    The way I see it is like this. Each SATA cable has 2 main lines (not including ground and the unused 3.3V line), 5V and 12V so the relay needs to be DPDT so that I can switch both of those at the same time. Each hard drive will only pull a max of 12V so the relay needs to be able to support at least 12VDC. Then from what I have been reading laptop drives pull about .5 to 1 amp max so the relay needs to be able to support about 1 to 1.5 amps to be safe (since only 1 hard drive will be on at a time). Also, the relay needs to be set to hard drive while its off, and one while it is on, so DPDT will work for that AND it doesn't need to be latching because when it looses power I want it to switch back to the other HDD.

    So basically:
    • DPDT
    • 3.3V Power
    • Supports at least 1.5amps @12VDC

    Does such a relay exist? I know that the higher the power you want to put through the actual lines that the relay controls, the higher the power needs to be for the relay itself. I also prefer it to be solid state because it would be smaller (space in this project is a slight issue). Also I have heard that laptop (2.5") drives only use 5V. Is that true? If so can I just drop the 12V line? Or keep it plugged in but only require the relay to have a rating of 5VDC?

    Thanks a bunch.
     
  2. #12

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  3. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Wow nice site. So it seems that my best bet is to get a resistor that has a high enough contact rating with a coil power of 3 volts, and use a resistor to take the source power from 3.3V to 3V.

    One other question though. I plan to use a diode in-between this and the power source for the coil and I read about something called "Diode Voltage Drop". Does that litterally mean that the voltage on the other side of the diode (the "out" if you will - the end that leads to the relay I will be using) is less than the other end? So would the voltage on the other side of the diode already be less than 3.3V
     
  4. #12

    Expert

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    Yes. A diode will cost about .6 to .7 volts DC.
    and get a plan for the inductive kick when you turn the coil off. It can blast right through a diode if you don't know what you're doing.

    Want advice on that? Post a schematic.
     
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  5. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Couldn't I use a Schotty Diode to make the drop less so that it's more about a .3 volt drop? Then I wouldn't even need a resistor. Also if I used a solid state relay will I still hbe the problem your talking about? I will post a schematic (a crapy noon version) once I pick a relay. Also for the relay is the wiring as simple as:
    1 Contact for the coil is the 3V source in and the other contact goes to ground

    Then since I'm going to use a DPDT relay the two middle contacts are the common input, and the two contacts on each sides of those the outputs?
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    It totally depends upon the design of the relay. You will have 8 contacts points for a DPDT relay. 2 for the coil, 2 for relay common, 2 for NC and two for NO. You will need to look at the data sheet for your particular relay to make sure of which is which.
     
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  7. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    I know that the contact layout will be different for each relay, but I just wanted to make sure I had the concept down which your post confirms. But just to double check, is it the same thing for SSRs? I thought I read that the wiring was different on them somehow but I don't know I couldn't really figure it out. I'll make a schematic when I get home from school just to confirm what I am doing will work.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  9. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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  10. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    OK so I made the diagram. Ground for the hard drives is just always closed (the purple and green) since it wont matter since only one hard drive will be on at a time, and I am not positive but I am pretty sure that sharing Ground is safe as long as both aren't on at the same time. Then the red cable is the 12V power and yellow cable is 5V power. The 12V line goes into contact 1 (red) of NO and NC for each hard drive respectively, and contact 1 of the Common contacts. The 5V line goes into contact 2 (blue) of NO and NC for each hard drive respectively and contact 2 of the Common contacts. That should take care of the configuration of the hard drives power lines.

    Then the relay itself will have a coil voltage (trigger voltage i guess) of 3V. I need a diode from the power source because there is another line coming off of the same source going somewhere else. I already checked with the manufacturers of the device I am drawing power from and they said it would be OK to take 3.3V from this point as long as I use a diode. Since the relay is 3V and the input is 3.3V a normal diode would drop too much voltage so I put in a Schotty diode which should get it to around 3V. Then the line out just goes to ground.

    Will this setup work? Will the power lines switch to HDD 2 when the relay is ON (circuit closed) and switch back to HDD 1 when the relay is OFF (circuit open)?

    Also, I was thinking of something else that I was wondering if it is an issue. Since the voltage drops of a diode aren't perfectly consistent and vary with temperature, will the relay still work if the current deviates slightly from 3V?

    Those of you who have had the patience to help me with this I thank you greatly!

    Diagram:
    [​IMG]
     
  11. #12

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    Last edited: Apr 3, 2013
  12. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Ok cool. I didnt know how much leeway there was with relays. Would a 1N1418 diode be fine? Otherwise the setup looks good?
     
  13. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Oh also, (and this goes with checking the whole setup anyway) is it safe that the hard drives share ground like that?
     
  14. #12

    Expert

    Nov 30, 2010
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    I can't even find a datasheet for a 1n1418 diode. Do you know what it is? Do you have a datasheet? Why not just use a 5 cent 1n4007?

    The setup doesn't look because nobody can see behind the diode and find out what's supplying it.

    Put another diode across the coil of the relay to absorb it's inductive discharge.
     
  15. #12

    Expert

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    They always have.
     
  16. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    Unfortunately I cannot tell you either because behind that point is a long path back to the source. All I know is that I was told that for what I am doing it should be fine to take power from there, and a 1n1418 switching diode was recommended (I will double check with why that was suggested - The most I could find on it is the wiki article: http://en.wikipedia.org/wiki/1N4148). It is known that the point gives out 0V while the device is off and a consistent 3.3V when the device is on, and I will test myself to make sure it will power the relay effectively. But other than the power for the relay itself does the diagram look good? Like the way that I have the hard drives 12V and 5V lines hooked up to the relay?

    Oh and how would I go about attaching a diode across the coil?

    EDIT: I just realized I said 1N1418 instead of 1N14148 sorry.

    EDIT2: The person recommened it because its "just a good fast switching diode" and there are a bunch of console mods that use it so people tend to have a bunch of leftovers, which I do.
     
    Last edited: Apr 4, 2013
  17. #12

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    Yes, your wiring diagram looks like it will work.
    1N4148 diodes will work.
     
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  18. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    I appreciate all of your help with this. You answered a bunch of my questions and covered all of my concerns. The only "doubt" there is now is if the relay will be properly powered, but I am confident it will be.

    Only thing to do now is try it out. I'll let you know how it goes I guess.

    That SSR you linked me to will be the one I try out.

    Also, dumb question, the 1N4148 will still have about .6 volts dropped right? Because 3.6V max (150% like you said) is somewhat close to 3.3V

    ALSO, I researched the thing you said about using a diode as reverse current protection and I get it now. You simply run a diode in parallel to the coil contacts facing in the opposite direction that the current is flowing. Will the voltage drop matter for the protection diode and can I just use another 1N4148
     
    Last edited: Apr 4, 2013
  19. #12

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    I'm really tired of this. I have handed you everything from the part number to the schematic, and you want to talk about it.

    Where's that thread about having more female members lately?

    IT'S THAT SIMPLE! ONE RELAY AND 2 DIODES. CONNECT THEM TOGETHER.
     
    Last edited: Apr 4, 2013
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  20. oblivioncth

    Thread Starter New Member

    Apr 2, 2013
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    LOL! (I'm a male so huh?) I am sorry for asking so many questions but I am only 18 and this is my first time using a relay or doing any soldering in which I have not followed directions or done something simple like replacing a USB port. I have no formal education in this field and just don't want to break anything. Everything I have learned is through the internet and experimentation. It just helps to talk to an actual person for confirmation. I am pretty sure I can figure out any more little details on my own and I am sorry for having this go on so long.

    Thanks again.
     
    Last edited: Apr 4, 2013
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